EP4095279A1

EP4095279A1 - Martensitic stainless steel sheet and martensitic stainless steel member

Info

Publication number: EP4095279A1
Application number: EP21744342.3A
Authority: EP
Inventors: Yoshiharu Inoue; Shinichi Tamura; Yoshihito Yamada
Original assignee: Nippon Steel Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2020-01-22
Filing date: 2021-01-15
Publication date: 2022-11-30
Also published as: TW202134450A; KR20220115621A; WO2021149601A1; TWI747722B; JP2023046414A; CN115003838B; CN115003838A

Abstract

A martensitic stainless steel sheet contains, in mass%: C: 0.100 to 0.170%; Si: 0.30 to 0.60%; Mn: 0.10 to 0.60%; Cr: 11.0 to 15.0%; Ni: 0.05 to 0.60%; Cu: 0.006 to 0.50%; V: 0.010 to 0.10%; Al: 0.05% or less; and N: 0.040% or more and less than 0.060%, where C+1/2N is 0.130 to 0.190% and γp represented by a formula (1) is 120 or more. When the martensitic stainless steel sheet is hardened and tempered, an area ratio in a sheet-thickness cross section of a δ ferrite (δFe) present in a sheet-thickness central part of the steel sheet is less than 0.1%.

Description

TECHNICAL FIELD

The present invention relates to a martensitic stainless steel sheet and a martensitic stainless steel member that are excellent in corrosion resistance after being hardened. More specifically, the present invention relates to a martensitic stainless steel that exhibits excellent corrosion resistance even after being hardened by air cooling, which is used for manufacturing tableware knives, looms, tools, disk brakes, and the like.

BACKGROUND ART

Martensitic stainless steel (e.g. SUS420J1 steel and SUS420J2 steel) sheet is typically used for tools such as tableware knives (table knives), scissors, looms, caliper gauges, and the like. In such applications, it is difficult to apply plating, painting, and rust-proof oil. Accordingly, rust resistance is required for the material in itself. Further, it is also important for the material to have high hardness due to the need for wear resistance.
The manufacturing steps of tableware knives and the like typically include punching from a steel sheet, heating, hardening, and, subsequently, polishing. The hardening step is often performed at a low cooling rate (e.g. air cooling) in view of the excellent hardenability of the martensitic stainless steel sheet.
Patent Literature 1 discloses a martensitic stainless steel excellent in corrosion resistance when being hardened by air cooling. In Patent Literature 1, approximately 0.06% of N is added as an element for improving corrosion resistance.
Patent Literature 2 discloses a steel added with a larger content of N than Patent Literature 1. Patent Literature 3 discloses a steel whose N content is further increased using special equipment.
Recently, requirements for corrosion resistance of tableware become increasingly strict mainly in Europe. Consequently, a blunt edge or cutting edge of a blade or a central part of handle of a table knife is sometimes found to be rusted in a rust resistance evaluation test, so that solution thereto has been requested.

CITATION LIST

PATENT LITERATURE(S)

Patent Literature 1 JP 2008-163452 A
Patent Literature 2 JP 2005-163176 A
Patent Literature 3 JP 2005-248263 A

NON-PATENT LITERATURE(S)

Non-Patent Literature 1 Journal of Japan Institute of Metals, 1962, vol. 26, No. 7, pp. 472-478

SUMMARY OF THE INVENTION

PROBLEM(S) TO BE SOLVED BY THE INVENTION

Recently, in accordance with the increasingly strict requirements for corrosion resistance of tableware mainly in Europe, a demand for reduction in the rust on a blunt edge or cutting edge of a blade or a central part of handle of a table knife in a strict corrosion resistance test is on the rise. An object of the invention is to provide a martensitic stainless steel sheet and a martensitic stainless steel member that is excellent in end-surface corrosion resistance while retaining hardness sufficient for the use of martensitic stainless steel for tableware (e.g. table knives) and the like.

MEANS FOR SOLVING THE PROBLEM(S)

In order to achieve the above object, the inventors investigated in detail on the rust generated on table knives. As a result, it was found that the rusted part was started from an end surface of a steel sheet, more specifically, a thickness central part of a steel sheet. Further, it was found that the rust was generated as follows: a δ ferrite phase (δFe phase) derived from macro-segregation was generated at the thickness central part of the steel sheet; then, the grain boundaries of the δFe formed accumulation sites of carbides; the carbides were melted by heat during the hardening step to be precipitated in grain boundaries during a subsequent cooling process; and, as a result, sensitization occurred to cause grain boundary corrosion.
It was also found that the rust generation was dependent on a cooling rate during the hardening step. The cooling rate, which greatly varies depending on hardening equipment, exceeds 100 degrees C/s in terms of average cooling rate from a hardening temperature to 600 degrees C (a temperature for the carbides to be almost completely precipitated) in water cooling during the hardening step, so that the carbide is restrained from being precipitated and the rust is unlikely to be generated. In contrast, the cooling rate by the air cooling typically used in the manufacturing steps of table knife is approximately 5 degrees C/s, so that the carbide precipitation cannot be restrained, thereby promoting rust generation.
As a result of studies made by the inventors based on the above findings on how to reduce the rust generated on the steel sheet, it is found that the rust generation in tableware knives after being subjected to forming and heat treatment can be reduced by adding N to steel sheet components and restricting the δFe amount in the steel, which is inevitably present in steel made by a typical process, at a predetermined amount through optimization of the manufacturing steps.
The invention is completed after subsequent further detailed studies.
Specifically, the following(s) is provided by some of the aspects of the invention.

(1) A martensitic stainless steel sheet of a steel composition containing, in mass%:
- C: 0.100 to 0.170%;
- Si: 0.25 to 0.60%;
- Mn: 0.10 to 0.60%;
- P: 0.035% or less;
- S: 0.015% or less;
- Cr: 11.0 to 15.0%;
- Ni: 0.05 to 0.60%;
- Cu: 0.006 to 0.50%;
- V: 0.010 to 0.10%;
- Al: 0.05% or less;
- N: 0.040% or more and less than 0.060%;
- C + 1/2N: 0.130 to 0.190%; and
- a balance consisting of Fe and impurities, in which
- γp represented by a formula (1) below is 120 or more, and
- when the martensitic stainless steel sheet is held in an environment at 1050 degrees C for 30 minutes, subsequently hardened by air cooling hardening, and tempered at 150 degrees C for 30 minutes, an area ratio in a sheet-thickness cross section of a δ ferrite (δFe) present in a sheet-thickness central part is less than 0.1%, $\begin{array}{l} γ p = 420 C + 470 N + 30 Ni + 7 Mn + 9 Cu - 11.5 Cr - 11.5 Si - 12 Mo - 23 V - 47 Nb \\ - 52 Al + 189 \end{array}$
- where atomic symbols in the formula (1) each represent a content (mass%) of each element.
(2) The martensitic stainless steel sheet according to (1), in which the steel composition contains, in place of a part of the Fe, in mass%, one or two or more of:
- Mo: 0.01 to 1.0%;
- Ti: 0.005 to 0.050%; and
- Nb: 0.005 to 0.050%.
(3) The martensitic stainless steel sheet according to (1) or (2), in which the steel composition contains, in place of a part of the Fe, in mass%, one or both of:
- Sn: 0.01 to 0.10%; and
- Bi: 0.01 to 0.20%.
(4) A martensitic stainless steel member of the steel composition according to any one of (1) to (3), in which
- γp represented by the formula (1) is 120 or more, and
- the area ratio in the sheet-thickness cross section of the δ ferrite (δFe) present at the sheet-thickness central part is less than 0.1%.

The martensitic stainless steel sheet of the invention exhibits, while retaining hardness sufficient for the use of martensitic stainless steel for tableware (e.g. table knives), excellent corrosion resistance (especially, end-surface corrosion resistance). Accordingly, when the martensitic stainless steel sheet is used for a martensitic stainless steel member (e.g. tableware knives), increase in product lifetime can be expected in addition to improvement in corrosion resistance.

BRIEF EXPLANATION OF DRAWINGS

Fig. 1 shows a representative example of a cross section structure of a steel sheet of the invention, which is etched using modified Murakami reagent.

DESCRIPTION OF EMBODIMENT(S)

The invention will be described below in further details.

Chemical Components of Steel Sheet and Steel Member (%: mass%)

C: 0.100 to 0.170%

C, which is an element that determines hardening hardness as with N, is required to be contained by 0.100% or more in order to achieve the hardness required for tableware knives. Preferably, the C content is 0.110% or more and 0.120% or more. In contrast, excessive addition of C increases hardening hardness more than necessary, resulting in increase in polishing load and decrease in toughness. In addition, even with the invention, Cr carbide is likely to be precipitated to impair corrosion resistance at the time of hardening by air cooling. Accordingly, the C content is 0.170% or less. Preferably, the C content is 0.155% or less.

Si: 0.25 to 0.60%

Si, which is an element necessary for deoxidation during steelmaking process and effective for restraining oxide scales generated after hardening thermal treatment, is contained by 0.25% or more. When the Si content is less than 0.25%, the oxide scales are excessively generated to increase final polishing load. However, when Si is excessively added, austenite is restrained from being generated to impair hardenability. Accordingly, the Si content is 0.60% or less.

Mn: 0.10 to 0.60%

Mn, which is an austenite stabilizer element, is necessary for ensuring hardness at the time of hardening and the amount of martensite. Accordingly, the Mn content is 0.10% or more. However, Mn promotes generation of oxide scales at the time of hardening to increase subsequent polishing load. Accordingly, the Mn content is 0.60% or less. Further, excessive addition of Mn results in a large amount of MnS and consequent reduction in corrosion resistance.

P: 0.035% or Less

P is an element contained in a form of impurities in a raw material that is hot metal or an alloy (e.g. ferrochrome). P, which is an element detrimental to toughness of a steel sheet after hot-rolled annealing and hardening, is contained by 0.035% or less. Excessive addition of P reduces hot workability and corrosion resistance.

S: 0.015% or Less

S, which has small solid solubility to an austenite phase, segregates in grain boundaries to promote reduction in hot workability. Further, excessive addition of S results in a large amount of MnS and consequent reduction in corrosion resistance. Accordingly, the S content is 0.015% or less.

Cr: 11.0 to 15.0%

In order to provide the corrosion resistance for tableware knife, Cr content is necessary to be at least 11.0% or more. In contrast, Cr has an effect of narrowing a range of austenite stability temperature. Accordingly, the Cr content is 15.0% or less. Preferably, the Cr content is 12.0% or more. A preferable upper limit is 14.0% or less. Preferably, the Cr content is in a range from 12.0 to 14.0%.

Ni: 0.05 to 0.60%

Ni, which is an austenite stabilizer element as in Mn, is necessary for ensuring hardness at the time of hardening and the amount of martensite. Further, Ni is effective for improving corrosion resistance. Accordingly, the Ni content is 0.05% or more. However, excessive addition of Ni may increase the stability of γ phase to decrease the amount of martensite. Further, since Ni is expensive as compared with other elements, the upper limit of the Ni content is 0.60%.

Cu: 0.006 to 0.50%

Cu, which is an austenite stabilizer element as in Mn and Ni, is also an element that improves corrosion resistance. Cu is also an element inevitably mixed into steel from scrap steel during the steelmaking process. In order to improve corrosion resistance, the Cu content is 0.006% or more. Preferably, the Cu content is 0.02% or more. Further preferably, the Cu content is 0.05% or more. In contrast, when Cu is excessively contained, hot workability and the like are deteriorated. Accordingly, the Cu content is 0.50% or less. Though being inexpensive as compared with Ni, Cu is a relatively expensive element and thus is preferably as low in content as possible.

V: 0.010 to 0.10%

V is an element often inevitably mixed from alloy elements of ferrochrome and the like. It is difficult to reduce the amount of V and too much load is applied during the steelmaking process. Accordingly, the V content is 0.010% or more. However, an excessive content of V narrows a range of austenite formation temperature. Accordingly, the V content is 0.10% or less. Further, excessive addition of V results in VN to fix N, thereby unfavorably reducing hardness and/or corrosion resistance.

Al: 0.05% or Less

Al, which is an element effective for deoxidation, generates soluble inclusions in a form of CaS during hot rolling to reduce corrosion resistance, when being excessively contained. Accordingly, the Al content is 0.05% or less. The Al content is preferably 0.001% or more. Al may not be contained.

N: 0.040% or More and Less Than 0.060%

N, which is an element that determines hardening hardness as with C and improves corrosion resistance, is an important element in the invention. Accordingly, the N content in the invention is 0.040% or more. Preferably, the N content is 0.045% or more. However, when N is excessively contained, air bubble defects are likely to be formed in a slab to adversely reduce corrosion resistance. In addition, N is an element that increases production cost in a secondary refining process (e.g. VOD). Especially, since it is difficult to stably produce steel without generating the air bubble defects by continuous cast, the N content is preferably as low as possible, which is less than 0.060%. Preferably, the N content is 0.057% or less.

C+1/2N: 0.130 to 0.190%

The elements that determine the hardness of the martensite phase in the steel are C and N, a sum of which contributes to the hardness. According to studies of the inventors, the contribution of N to the hardness is half of the contribution of C. Accordingly, in order to obtain the hardness necessary for tableware knife, it is necessary that C + 1/2N is 0.130% or more. Preferably, C + 1/2N is 0.150% or more. In contrast, when C +1/2N is excessive, the hardening hardness is so large that toughness of a product and/or an intermediate material (e.g. cast steel) during the manufacturing steps is impaired. Accordingly, C +1/2N is 0.190% or less. Preferably, C +1/2N is 0.180% or less, also preferably 0.175% or less.
Further, in order to exhibit stable hardness after being hardened, the components have to be adjusted with each other so that γp defined by the formula (1) is 120 or more. When γp is less than 120, the hardness can greatly vary depending on hardening conditions. Further, δFe in the steel increases. In the invention, γp is optionally adjusted to be 130 or more, or 140 or more. In the invention, γp is optionally 170 or less, or 150 or less.
The steel composition of the invention includes the above components and the balance consisting of Fe and impurities.
In addition to the above-described elements, the steel composition of the invention optionally includes, in place of a part of Fe, Mo, Nb, Ti, Sn, and Bi in order to improve rust resistance and corrosion resistance.

Mo: 0.01 to 1.0%

Mo, which is an element for improving corrosion resistance, achieves its effect when being added by 0.01% or more. However, Mo is an expensive element and clear effect cannot be achieved by adding excessive amount. Accordingly, the upper limit of the Mo content is 1.0%.

Ti: 0.005 to 0.050%

Ti is an element that forms a carbonitride to restrain sensitization and decrease in corrosion resistance that are caused by precipitation of chromium carbonitride in stainless steel. The above effect is achieved at a content of 0.005%. However, excessively added Ti destabilizes the martensite phase to reduce hardness. Accordingly, the upper limit of the Ti content is 0.050%.

Nb: 0.005 to 0.050%

Nb is an element that forms a carbonitride to restrain sensitization and decrease in corrosion resistance that are caused by precipitation of chromium carbonitride in stainless steel. The above effect is achieved at a content of 0.005%. However, excessively added Nb destabilizes the martensite phase to reduce hardness. Accordingly, the upper limit of the Nb content is 0.050%.

Sn: 0.01 to 0.10%

Sn, which is an element effective for improving corrosion resistance after hardening, is preferably added by a content of 0.01% or more, and by a content of 0.05% or more as necessary. However, excessively added Sn promotes edge cracking during hot rolling. Accordingly, the Sn content is preferably 0.10% or less.

Bi: 0.01 to 0.20%

Bi is an element that improves corrosion resistance. Though the mechanism is not clearly known, it is believed that added Bi, which is capable of reducing the size of MnS that is likely to be a start point of rust generation, decreases the probability for creating the rust generation start points. The above effect is achieved by addition of 0.01% or more. Adding Bi at a content exceeding 0.20% only saturates the effect. Accordingly, the upper limit of the Bi content is 0.20%.

δ Ferrite Phase Ratio of Steel Sheet and Steel Member

The inventors have found that δ ferrite (δFe) present at a sheet-thickness central part of a steel sheet greatly affects an end-surface corrosion resistance of the steel sheet. When a steel sheet is hardened by air cooling or the like with low cooling rate, it is believed that grain boundaries between δFe and a matrix phase (γ phase) become a precipitation site for Cr carbide during the cooling process, causing sensitization near the precipitated Cr carbide, thereby decreasing the end-surface corrosion resistance. It is also speculated that N improves the end-surface corrosion resistance because N restrains the precipitation of the Cr carbide.
Thus, it is effective in the invention, in addition to the N content, to reduce δFe that is present at the sheet-thickness central part of the steel.
The steel sheet of the invention is a steel sheet before being hardened. It would be favorable if δFe present in the steel sheet before being hardened could be measured. However, since all of the phases around δFe are ferrite phases, it is difficult to measure the δFe. In contrast, the structure around the δFe present in the steel sheet after being hardened/tempered is a martensite phase, which is relatively easily measured. Accordingly, the δFe amount in the (before being hardened) steel sheet of the invention is evaluated after subjecting the steel sheet to hardening and tempering processes. The hardening was performed by heating to 1050 degrees C, holding the temperature for 30 minutes, and cooling the steel sheet by air cooling. The tempering was performed at 150 degrees C for 30 minutes. When the hardening temperature is excessively low and/or the hardening time is excessively short, the remaining ferrite phase cannot be unfavorably distinguished from the δFe phase. In contrast, when the hardening temperature is excessively high and/or the hardening time is excessively long, the δFe phase is unfavorably transformed into a phase different from an initial phase. The steel sheet is hardened by air cooling. A steel sheet is subjected to hardening and tempering under the above evaluation conditions. Then, a presence area ratio of a δFe layer (δFe amount) in a sheet-thickness cross section of the steel sheet is evaluated.
As described above, the N content in the steel sheet and steel member of the invention is not at a sufficiently high level. Accordingly, it is necessary to further reduce δFe as compared with a steel sheet with larger N content. At the N content of the invention, excellent end-surface corrosion resistance can be achieved when the δFe amount is less than 0.1%. The corrosion resistance is increased as the δFe amount is reduced. However, the reduction in the δFe amount requires long thermal treatment as described below. Accordingly, the δFe amount is 0.05% or more. The δFe amount is preferably in a range of 0.05% or more and less than 0.1%.

Manufacturing Method of Steel Sheet

A typically employed method is used as the manufacturing method of the steel sheet of the invention. Specifically, a slab whose components are adjusted is obtained through melting and casting processes. Then, the slab is hot rolled, box-annealed, shot-blasted, and pickled to produce a steel product.
It should however be noted that the slab is preheated in order to regulate the δFe amount at a level of less than 0.1%. δFe can thus be generated in a less amount as compared with typical instances, thus achieving excellent end-surface corrosion resistance. The preheating is preferably performed under a temperature from 1100 to 1150 degrees C for a soaking time of more than 50 hours to 100 hours or less. When the heating temperature exceeds 1150 degrees C, two phases (γ + δ) become stable, where the δFe amount is unfavorably rapidly increased. Further, a large amount of the rapidly increased δFe remains in subsequent steps, which may be a factor for decreasing the hardness. In contrast, when the heating temperature is less than 1100 degrees C, δFe unfavorably does not decrease even after heating for a long time. The δFe amount is smaller than in an instance where the slab is heated at a temperature exceeding 1150 degrees C. Accordingly, the hardness can be retained depending on subsequent steps. When the soaking time is 50 hours or less, the δFe amount is unfavorably excessive. In contrast, when the soaking time exceeds 100 hours, the production cost unfavorably increases.
The preheating is optionally performed by heating the slab before being hot-rolled, and directly followed by the hot-rolling.

Manufacturing Method of Steel Member

In the invention, the produced steel sheet is punched and the punched sheet is hardened, tempered, and polished to produce a steel member. The punched steel sheet before being hardened is optionally forged for reshaping. It should be noted that preferable conditions for hardening and tempering are as follows. The hardening temperature is preferably in a range from 1000 to 1150 degrees C. When the hardening temperature is less than 1000 degrees C, there is a less austenite phase during a high-temperature period, so that the hardness after being hardened is unfavorably small. In contrast, when the hardening temperature exceeds 1150 degrees C, the δ phase and stable austenite phase increase, so that the hardness also unfavorably decreases. The holding time during the hardening step is preferably in a range from one minute to an hour. When the holding time is less than one minute, there is a less austenite phase during a high-temperature period, so that the hardness after being hardened is unfavorably reduced. In contrast, when the holding time exceeds one hour, the stable austenite phase increases, so that the hardness also unfavorably decreases. The cooling rate during the hardening step is preferably 1 degree/sec or more in terms of an average cooling rate from a hardening temperature to 600 degrees C. If the cooling rate is less than 1 degree/sec, the hardness unfavorably decreases. The air cooling used in the hardening step can achieve the above favorable cooling rate. The tempering temperature is preferably in a range from 100 degrees C to 250 degrees C. The tempering temperature of less than 100 degrees C is insufficient for achieving the tempering effect. In contrast, when the tempering temperature exceeds 250 degrees C, the hardness unfavorably excessively decreases.

Steel Structure of Steel Sheet and Steel Member

As described above, the steel sheet of the invention is produced by subjecting the slab to hot rolling, box-annealing, shot-blasting, and pickling. Further, the produced steel sheet is punched and the punched sheet is hardened, tempered, and polished to produce the steel member of the invention. Specifically, the steel sheet of the invention is a steel sheet before being hardened, whose steel structure has a crystal structure mainly made of ferrite and the like. In contrast, the steel member of the invention, which is produced by subjecting the processed steel sheet to hardening and tempering, has a steel structure mainly made of martensite. Steel with the above-described properties is usually referred to as "martensitic stainless steel sheet" even when the steel sheet is before being hardened, where the steel structure is mainly made of ferrite. In view of the above circumstances, the steel sheet of the invention is also described as a "martensitic stainless steel sheet."

Examples

Effects of the invention will be described below with reference to Examples. It should however be noted that the scope of the invention is by no means limited by conditions used in Examples below.
In Examples, steels of compositions shown in Tables 1 and 2 were melted to be casted into 250-mm thick slabs. Subsequently, these slabs were subjected to thermal treatment (preheating) at 1150 degrees C for 60 hours to set the δFe amount within a predetermined range. It should be noted that A2 steel was subjected to two different preheating processes (1175 degrees C for 60 hours, and 950 degrees C for 60 hours) to produce A2' steel and A2" steel.
Then, the slabs were heated to 1150 degrees C and were subjected to hot rolling to produce hot-rolled steel sheets having a 3 to 8 mm sheet-thickness. Subsequently, the hot-rolled steel sheets were annealed (box annealing). The maximum heating temperature was set in a temperature range from 800 degrees C to 900 degrees C. The surfaces of the annealed steel sheets were shot-blasted to remove scales and were pickled.

Example 1

In order to evaluate the produced steel sheets, evaluation samples were cut from the steel sheets. Then, after the samples were heated to and held at 1050 degrees C for 30 minutes (hardening and tempering), the samples were air-cooled and tempered at 150 degrees C for 30 minutes to produce steel members. Subsequently, the δFe amount and hardness were measured and end-surface corrosion resistance was evaluated on each of the steel members. The results are shown in Table 3.
The δFe amount was measured on an end surface of a sample that was mirror-polished and etched to expose structures thereon. The etching liquid, which may be aqua regia or the like for the purpose of exposing δFe, is preferably the modified Murakami reagent disclosed in Non-Patent Literature 1 that can etch δFe in deep brown. Accordingly, the modified Murakami reagent was used for evaluation. Fig. 1 shows a typical example of etched surface.
The structure exposed by the modified Murakami reagent was inspected with a microscope and a picture of δFe of a predetermined width (2 mm in Example 1) in a thickness direction was taken. The taken picture was analyzed to calculate an area of δFe, based on which an area ratio ([δFe area (mm²)/(2 mm × total thickness (mm))] × 100(%)) was calculated to obtain the δFe amount. In order for the steel member having the components within the composition range of the invention to exhibit excellent corrosion resistance, the value of the area ratio is necessary to be less than 0.1%. More preferably, the value is in a range of 0.05% or more and less than 0.1%. Samples with a δFe area ratio (δFe amount) of less than 0.1 % were evaluated to be acceptable (A) and samples with a δFe area ratio of 0.1 % or more were evaluated to be unacceptable (X).
The hardness (hardening hardness) was evaluated on a surface of the sample, which had been #80 polish-finished, in accordance with JIS Z 2245 using C scale Rockwell hardness tester. The hardness of 50 or more was evaluated to be acceptable (A), and the hardness of less than 50 was evaluated to be unacceptable (X).
In order to evaluate the end-surface corrosion resistance, after the surface and end surface of each of the samples were #600 polish-finished, each of the samples was placed so that the end surface (evaluated surface) faced upward. Then, salt spray test was performed for 24 hours (JIS Z 2371 "salt spray test method") to count the number of rusted point(s) formed on the end surface. 2 or less rust points were evaluated to be acceptable (A) and rust points exceeding 2 were evaluated to be unacceptable (X). Especially, samples with no rust point was evaluated to be acceptable (S). It should be noted that the rust is unlikely to further develop even after the salt spray test is performed for 24 hours or more. Accordingly, the end-surface corrosion resistance is determined based on the results of the salt spray test performed for 24 hours.
All of the samples prepared by applying the predetermined hardening and tempering on the steel sheet of the invention (corresponding to the steel member of the invention) are found to be excellent not only in end-surface corrosion resistance but also in other properties. Thus, they are suitable for steel sheet for tableware knives. In contrast, steels of Comparative Examples are inferior in end-surface corrosion resistance and/or other properties. Thus, they are clearly not suitable for steel sheet for tableware knives.

Example 2

A member cut from the produced steel sheet was hardened and tempered under the conditions shown in Table 4 to prepare a steel member. During the hardening step, the member was heated at a temperature ranging from 1050 to 1150 degrees C and was controlled to be cooled at a cooling rate shown in Table 4 from the hardening temperature to 600 degrees C. Further, the member was subjected to a tempering process at a temperature ranging from 150 to 250 degrees C for a period ranging from 1 to 2 hours to produce the steel member. The A2' steel and A2" steel were prepared in the same manner.
For the prepared steel members, measurements of the δFe amount and hardness, evaluation results of the end-surface corrosion resistance, and thermal treatment conditions are shown in Table 4. It should be noted that the method and standards of the evaluation are the same as those in Example 1.
All of the steel members of the invention, which are excellent not only in the end-surface corrosion resistance but also in other properties, are suitable for the steel member for tableware knives. In contrast, steels of Comparative Examples, which are inferior in end-surface corrosion resistance and/or other properties, are clearly not suitable for the steel member for tableware knives.
The invention allows efficient production of a martensitic stainless steel member and its material (martensitic stainless steel sheet) that are excellent in end-surface corrosion resistance after being hardened by air cooling and improvement in corrosion resistance of tableware knives manufactured using the steel member, which is very industrially effective.

Claims

A martensitic stainless steel sheet of a steel composition comprising, in mass%:
C: 0.100 to 0.170%;

Si: 0.25 to 0.60%;

Mn: 0.10 to 0.60%;

P: 0.035% or less;

S: 0.015% or less;

Cr: 11.0 to 15.0%;

Ni: 0.05 to 0.60%;

Cu: 0.006 to 0.50%;

V: 0.010 to 0.10%;

Al: 0.05% or less; and

N: 0.040% or more and less than 0.060%,

C + 1/2N being in a range from 0.130 to 0.190%, and

a balance consisting of Fe and impurities, wherein

γp represented by a formula (1) below is 120 or more, and

when the martensitic stainless steel sheet is held at 1050 degrees C for 30 minutes, subsequently hardened by air cooling, and tempered at 150 degrees C for 30 minutes, an area ratio in a sheet-thickness cross section of a δ ferrite (δFe) present in a sheet-thickness central part is less than 0.1%, $\begin{array}{l} γ p = 420 C + 470 N + 30 Ni + 7 Mn + 9 Cu - 11.5 Cr - 11.5 Si - 12 Mo - 23 V - 47 Nb \\ - 52 Al + 189 \end{array}$

where atomic symbols in the formula (1) each represent a content (mass%) of each element.
The martensitic stainless steel sheet according to claim 1, wherein the steel composition further comprises, in place of a part of the Fe, in mass%, one or two or more of:
Mo: 0.01 to 1.0%;

Ti: 0.005 to 0.050%; and

Nb: 0.005 to 0.050%.
The martensitic stainless steel sheet according to claim 1 or 2, wherein the steel composition further comprises, in place of a part of the Fe, in mass%, one or both of:
Sn: 0.01 to 0.10%; and

Bi: 0.01 to 0.20%.
A martensitic stainless steel member of the steel composition according to any one of claims 1 to 3, wherein
γp represented by the formula (1) is 120 or more, and

the area ratio in the sheet-thickness cross section of the δ ferrite (δFe) present at the sheet-thickness central part is less than 0.1%.